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Erik, Ug is the geostrophic wind velocity of the air in the x direction on an isobaric surface and delta Z/delta Y is the rate of change of height along the isobaric surface along the y direction. Small g of course is the acceleration due to gravity and f, the coriolis force. Vg = (g/f) (delta Z/delta X) is the same equation for the geostrophic velocity of the air in the y direction. I hope this helps a little. I would recommend the textbook by Hess called "Introduction to Theoretical Meteorology", which does a very excellent job of describing these components of the geostrophic wind on a constant pressure surface. I used this text when I was a student.